Palaumysis simonae Bacescu

Hanamura, Yukio & Kase, Tomoki, 2002, Marine cave mysids of the genus Palaumysis (Crustacea: Mysidacea), with a description of a new species from the Philippines, Journal of Natural History 36 (3), pp. 253-263 : 255-257

Palaumysis simonae Bacescu and IliOEe, 1986

(®gures 1, 2)

Palaumysis simonae Bacescu and IliOEe, 1986b: 31, ®gure 2.

Material examined

PARATYPES ( USNM 227132 ) : 12 males (1.2±1.4 mm), ®ve females (1.5±1.6 mm), one juvenile (1.0 mm), cave, 10 m, Caroline Island , Palau Islands .

Others. Seven males (ca 1.0± 1.4 mm), four females (1.3-1.5 mm), St. VH,`Virgin Hole’, W Ngemelis Island , Palau Islands , 07ss07¾18.5 ² N 134ss14¾14.7 ² E, 17 m, submarine cave, totally dark inside, calcareous muddy sand, 23 December 1999, leg. Ohashi, Kinjo , Hayami and Kase ( NSMT Cr 13082). One male (1.4 mm), St. NK,`Nikko Cave’, Palau Islands, 07ss08¾49.0 ² N 134ss09¾56.4 ² N, 12±15 m, submarine cave seemingly connected with an anchialine pond, totally dark inside, mud, 26 December 1999, leg. Ohashi, Kinjo, Kase, Tabuki and Kano ( NSMT Cr 13083) .

Description. Carapace reduced, fused with anterior three thoracic somites, without unfused free lobule posteroventrally, consequently leaving uncovered last ®ve thoracic somites as well as ventrolateral parts of anterior three thoracic somites; anterior margin produced into sub-triangular rostral plate with obtuse apex; anterolateral part forming acute angle; cervical groove distinct (®gure 1a, d, e).

Telson sub-triangular, with pair of distal spines, without plumose setae on all margins. Uropod narrow, exopod shorter than endopod, armed with three lateral setae and numerous long setae on distal and distomesial margins; endopod with long setae distally; statocyst present on proximal part of endopod.

Antennule with last segment of peduncle robust and longest, armed with three to ®ve, usually four, sensory setae at base of outer ¯agellum, male lobe undeveloped, inner ¯agellum composed of four articles (®gure 2a, c).

Antennal scale rudimentary, with minute terminal seta, latter hardly seen under light microscope (®gures 1b, 2a, b).

Labrum somewhat produced anteriorly, without frontal spine or process (®gure 1c).

Endopods of thoracic limbs 3±8 rather stout, carpo-propodi unsegmented.

Male pleopods rudimentary, unsegmented; pleopod 1 with long, plumose terminal seta, curving anteriorly, and few short setae distally (®gure 1f); pleopod 4 armed with a few lateral setae, ending with stout terminal spine, distal end slightly exceeding beyond posterior margin of abdominal somite 5; pleopods 2, 3 and 5 similar in shape (®gures 1g, 2d). Female pleopods tiny, unsegmented; pleopod 1 with procurved long, plumose terminal seta in immature females, but normally absent in mature females; pleopods 2±5 similar in shape, armed with about three setae.

Lamellar epipod on thoracic limb 1.

Female marsupium consisting of two pairs of lamellae.

Colour

The entire body is assumed to be semi-transparent as the foregut can be seen through the exoskeleton, and the uropod and distal parts of thoracic limbs with light brownish setae (based on specimens about one month after preservation in 10% seawater formalin).

Body size Total length does not exceed 2 mm, and females become larger than males.

Remarks

Bacescu and IliOEe (1986b) did not give detailed accounts for the general form of the carapace of Palaumysis simonae . It is very remarkable for its reduced size, covering only the dorsal and dorsolateral parts of the anterior three thoracic somites, and is completely fused with the anterior thoracomeres, in consequence with no unfused free lobe posteroventrally.

The original authors mentioned that the pleopods of Palaumysis simonae do not diOEer between the sexes, in that females have a long terminal spine as males do (Bacescu and IliOEe, 1986b; ®gure 2f). Although one paratype female was recognized as possessing such an end-piece, the remaining females did not bear it. This suggests that the presence of the terminal spine in females represents an aberrant condition. The ®rst pleopod, otherwise, has a procurved long seta in males and immature females, but it is not present in mature females.

For distinction from the Philippines population, see the`Remarks’ section under the following species.

Distribution Palau Islands; 10±24 m (Bacescu and IliOEe, 1986b; present study).